Predict your next period date, fertile window, and ovulation day for the next 6 months. Based on your last period date and average cycle length. Accurate menstrual cycle tracking for period prediction, family planning, and cycle awareness. Free, instant, private — no sign-up needed.
Next period, fertile window & ovulation — 6-month forecast
The average menstrual cycle is 28 days, but research from the Apple Women's Health Study (one of the largest US menstrual health studies, with data from over 60,000 participants) shows that only 13% of cycles are exactly 28 days. The normal range is 21–35 days with a period duration of 2–7 days. Cycle length can vary by up to 9 days from month to month even in otherwise healthy women — making accurate tracking important. Ovulation typically occurs 12–16 days before your next period (not 14 days after your last period as commonly stated) — this is why the "fertile window" shifts for women with longer or shorter cycles.
Period tracking provides valuable health insights beyond family planning. Irregular periods (outside the 21–35 day range or varying by more than 9 days) can indicate conditions like PCOS (polycystic ovary syndrome), thyroid dysfunction, perimenopause, or high stress levels — all of which warrant evaluation by a healthcare provider. The American College of Obstetricians and Gynecologists considers the menstrual cycle a "vital sign" — as important as blood pressure for evaluating reproductive and overall health. Apps and calculators like this one support cycle awareness but are not a substitute for contraception — typical use effectiveness of the fertility awareness method is 76–88%.
Menstrual phase (days 1–5): uterine lining sheds. Follicular phase (days 1–13): estrogen rises, follicles develop. Ovulation (day 14 avg): egg released from dominant follicle. Luteal phase (days 15–28): progesterone rises to prepare for potential implantation, then drops triggering next period if no pregnancy occurs.
Sperm survive 3–5 days in the female reproductive tract. An egg is viable for 12–24 hours after release. The fertile window is therefore 5–6 days before ovulation through the day of ovulation. This calculator estimates the fertile window as days 10–17 of a 28-day cycle (adjusted for your cycle length). For conception, timing intercourse in this window maximizes chances.
See your OB-GYN if: periods are consistently outside the 21–35 day range; you bleed for more than 7 days; your flow is extremely heavy (soaking a pad/tampon hourly for several hours); you experience severe cramping (dysmenorrhea); your period stops for 3+ months (amenorrhea) without pregnancy; or your cycle varies by more than 9 days month to month.
Consistent tracking over 3+ months provides the most accurate predictions. Log: start date, end date, flow level, and any symptoms (cramping, mood changes, spotting). After 3 months, your average cycle length becomes clear. Stress, illness, significant weight changes, and new medications can all temporarily disrupt cycle regularity.